Control system for electric furnaces



F. THORNTON, JR. CONTROL SYSTEM FOR ELECTRIC FURNACES.

APPLICATION FILED mm. 19, I919.

1,349,391 Patented Aug. 10, 1920.

:Lip' 1s 17 T T m I WITNESSES: INVENTOR ATTORNEY UNITED STATES PAEENT OFFICE.

FRANK THORNTON, JR, 013 MANSFIELD, OHIO, ASSIGNOB T0 WESTINGHOUSE ELEC- TRIO & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA.

CONTROL SYSTEM FOR ELECTRIC FURNACES.

I Specification of Letters Patent.

Patented Aug. 10, 1920.

Application filed March 19, 1919. Serial No. 283,624.

To all whom it may concern:

' Be it known that I FRANK THORNTON, Jr., a citizen of the limited States, and a resident of Mansfield, in the county of Richland' andState of Ohio, have invented a new and useful Im rovement in Control Systems for Electric urnaces, of which the following is a specification.

My invention; relates to electric furnaces and particularly to methods of control for electric-resistance furnaces.

()ne object of my invention is to provide an automatic system of control for an electric-resistance furnace where the resistor has a negative-temperature-resistance coefiicient.

Another object of my invention is to provide an automatic control system for electric-resistance furnaces that shall be simple and easily adjusted and that may be kept in working order by the workmen operating such furnaces.

In operating electric-resistance furnaces that have a negative-temperatureresistance coefficient, it is necessary to apply ahigher voltage to the resistor when cold than is required when the resistor is hot. Where a transformer is used to reduce the line voltage to the value required by the furnace, this variable voltage may be obtained by progressively changing the number of effective turns in the primary winding of the transformer, andmy invention: relates to methods of accomplishing this.

In ractising my invention, I provide a trans ormer having a plurality of spaced taps on the primary winding and a plurality of electrically operated switches to control the connection of the supply circuit to the primary winding, actuating these switches by a corresponding number of doubleecontact,double-coil relays. These relays are, in turn, controlled by-a corresponding number of double-contact, quick-acting thermostats that are caused to operateb a plurality of heating coils of different heating capacities. These heating coils are connected in series and to a-source .of electromotive force so arranged as to send a current through the heating coils that is, directly proportional to the current in the resistor of the furnace. This sourceofelectromotive force may be a current transformer or a series resistance of suitable value connected to av conductor carrying the current traversing the resistor. The relays are electrically interlocked so that not-more than one switch may be closed at one time.

The single figure of the drawing is 9. diagrammatic view of apparatus and circuits embodying my invention. As shown in the drawing, a supply circuit 10 energizes the primary winding of a transformer 20 through a conductor 11 connected to one of the conductors of circult l0, and selectively controlled conductors 12, 13, or 14, a suitable switch (not shown) being used to interrupt the circuit when the furnace is not in operation. The conductors 12, 13 and 14 terminate at corresponding contact terminals of the electrically operated'switches 17, 16 and 15, respectively, the other OOITQ" sponding contact terminals of these switches being connected to a common return conductor 18 that is connected to the other conductor of the supply circuit 10.. The f secondary winding of the transformer 20" is divided into halves that may be connected in series or in parallel by a suitable switch 19 and energizes the furnace 21 through two conductors 22 and 23.

A control supplycircuit 24 is provided to energize the coils of the electrically operated switches 15, 16 and 17, the circuits through these coils being controlled by double-contact, double coil relays 25, 26 and 27. Corresponding terminals of-the coils of switches. 15, 16 and 17 are connected to conductor 24 and the other corresponding terminals to conductors 28, 29 and 30, respectively. Conductor 28 terminates in an adjustable contact terminal 31 of the relay 25, and, similarly conductors 29-and 30 terminate, respectively, in adjustable con tact terminals 32 and 33 of the relays 26 and 27. As relays 25, 26 and 27 are alike, a.

description of the construction of relay"25' will sufiiee for the three. Two coils, 34 and 35, are placed a relatively small distance apart, with their axes in alinement. Two

soft iron cores, 36 and 37, are operatively connected together by a. metal rod' 38 of such length t at, when the core 37 is wholly within coil 35, the core 36' is partially withdrawn from coil 34, A metal bar 39, is ivotally connected to a stud or pin 40, and

as a slot 39 near its other end. A. in 38,

secured to rod 38 and projecting-t rough the slot 39", serves to actuate the free end of bar 3. A contact lever 41 is also pivot-c.

of its turning movement, when it will cause contact lever 41 to turn relatively quickly on its pivot stud 40, thus providing a quickbreak switch.

One terminal of coil 34 is connected to an adjustable contact terminal 43 by means of a conductor 44, and, similarly, one terminal of a coil 45 is connected to,an adjustable contact terminal 49 by means of a conductor 51, and one terminal of a coil. 47 is connected to an adjustable contact terminal 50, by means of a conductor 52. The other terminalscf coils 34, 45 and- 47 are connected, respectively, to contact terminals 65, 68 and 71 of thermostats 66, 70 and 73, by means of conductors 56, 57 and 58.. One terminal of coil is connected to conductor 28 by means of a conductor 59. The corresponding terminal of coil 46 is connected to conductor 29, b

means of a conductor 61, and the correspon ing terminal of coil '48 is connected to conductor 30, by means of a conductor 63; The other terminal of coil 35 is connected to a contact terminal 67 of a thermostat 66, by

means of a conductor 60. The correspond mg terminal of coil 46'is connected to a contact terminal 69 of a thermostat 70, by means of a conductor-62, and thecorresponding terminal of a coil 48 is connected to the contact terminal 72 of a thermostat 73, by meansof a conductor 64. The contact lever 41 is connected to one of the terminals of coil 45, by means of conductor 53, and a contact lever 80 is connected to the corresponding terminal of coil 47, by means of a conductor 54, and a contact lever 81 is connected to conductor 24 of the control circuit 24 by means of a conductor 55.

iDouble-contac't thermostats 66, and 73 are provided, with their stationary contact terminals connected as stated above, while the fixed end of bi-metallic strips 66, 70 and 73 are connected to a conductor 7 4 that is connected to conductor 24* of the control circuit 24. The strips 66, 70 and 7,3 engage the contact terminals 65, 68 and T1, respectively, when cold, and, when heated, become disengaged therefrom and imme liately engage contact terminals 67, 69 and 72, respectively. Heating coils 66", 70 and 73', that are associated with the respective thermostats 66, 70 and 73, are connected in series and are energized by the secondary coil 76: of a current transformer connected to. the'conductor 22. If desired, the current transformer may be replaced by a series resistance, as the object of this part of the device is to cause a current to flow through the heating coils that is in direct proportion to the current flowing through the resistor of the furnace. Coil 73 has the greatest watts loss of the three coils, and coils 70" and 66 have each a progressively smaller watts loss. A rheostat 79 is provided inshunt, with. the heating coils so as to enable the operator to divert part ofthe current and thus control the working temperature of the furnace, as will hereinafter e more fully described.

Assuming that the position of the various parts is as shown in the drawing, if it is desired to place the control system in operation, the proceeding is as follows: close switch 19 to connectthetwo halves of the secondary winding of transformer 20 in parallel and energize circuits 10 and 24 by closing the,respective control switches (not shown in the dia ram). A circuit will be established as fol ows: from conductor 24 of circuit 24, through conductor 74, strip 73", contact-terminal 71, conductor 58, coil 47, and thence, through conductor 52, contact terminal 50, lever 81, conductor 55, to conductor 24 of circuit 24. A second circuit will be established, as follows: from conductor 24, conductor 74, strip 70*, contact terminal 68, conductor 57 coil 45, and thence, through conductor 51, contact terminal '49, lever 80, conductors 54 and 52, contact terminal 50, lever 81, conductor 55, to conductor 24" of circuit 24. A third circuit comprises conductor 74, strip 66, contact terminal 65, conductor 56, coil 34, conductor 44, contact terminal 43, lever 41, conductors 53 and 51, contact terminal 49, lever 80, conductors 54 and 52, contact terminal 50, lever 81 and conductor 55, to conductor 24 of circuit 24. These circuits will eners gize the threecoils 34, 45 and 47, causing them to attract vtheir respective soft iron cores, and will operate to give the contact levers 41, 80 and 81 a turning movement on their respective pivots, and the free ends of these levers will engage the respective contact terminals 31, 32 and 33.

A control circuit will be established, through the operating coil of switch 17, as follows: from conductor 24 of circuit 24, through the coil of switch 17, conductor 30, contact terminal 33, lever 81, and conductor 55, to conductor 24 of circuit 24. This will cause switchl'i' to operate and establish a circuit, as follows: from one conductor of circuit ..10, through conductor 18, the contact terminals and contact bridging member of switch 17, conductor 12, a part of the primary winding of the transformer 20 and conductor 11, to the other conductor of circuit 10. As a relatively small number of turns of the primary winding of transformer 20 is included in the circuit, the secoiidaryvoltage 1 ofthe transformer will. be relatively highand-will cause a current to flow through the cold resistor of the furnace 21. The temperature of the resistor will .increase and the resistance will, therefore, decrease, 'resulting'in an increase in the current-value. *Anelectromotive force will be induced in thesecondary winding 76 of the transformer 75thatwill cause a current to" flow 'through'the heating coils 66 70 and 73 in'series' and also through the rheostat 79 if s'aine is connected in circuit. its heatin'g' coil 7 3 has'the. largest watts loss of the three coils, the thermostat'strip 73 will be causedto'move first andwill bedisengaged from its contact' terminal 7l' and immediofcircuit24, This will cause-coil 48 to at tract its coiiperatin'gg core member and will result in contact lever 81' having a turning movement, causingits free'end to engage contact-terminal 50.- A new: control circuit is thereby'established, as follows: from conductor 24 of circuit '24, through the coil of switch 16,conductor 29, contact terminal 32, lever 80, conductors 54 and 52, contact terminal 50, lever 81'and conductor 55, to the conductor- 29 0f circuit 24. This will cause switch 16 to operate and close a circuit, as follows: from one conductor of circuit 10, through conductor-18, the contact terminals and contact-bridging member of switch 16, conductor '13, the primary winding of transformer 20 and conductor 11, to the other conductor of the supply circuit 10. A relatively large number of turns of the primary winding will, therefore, be included in the circuit, and thesecondary voltage impressed 'on the resistor of the furnace '21 will hie-correspondingly decreased.-- Y :4

If the resistance of the furnace 21continues to decrease, with a corresponding ,increase of current throughit and through the conductor 22, the derived current throu h the'secondary winding 76 and through t e heating coils 66, 70" and 73 will increase and, at a predetermined value of current, the amount of heat generated in the coil 70" will 'cause the strip 70* to be disengaged from its cooperating contact terminal 68 and to immediately engage contact terminal 69. A circuit will thereby be established, as follows: from conductor 24 of circuit 24, through conductor ,74, strip 70*, contact terminal 69, conductor 62, coil 46, conductors 61 and 29,-contact terminal 32, lever 80, conductors 54 and 52, contact terminal 50, lever 81 and conductor 55, to conductor 24 of circuit 24. This will cause coil 46 to attract its cooperating core member and cause the contactlever 80 to be disengaged from contact terminal 32 and immediately to engage contact terminal 49, This opera tion-serves to interrupt the circuit through the'coil of switch 16, thereby causing said switch to open and interrupt the circuit through theprimary winding of trans-' former20 and denergize the same. I

A new control circuit has been established by the turning movement of lever 80, as follows: froni'conduc'tor '24 of circuit-=24, through the coil of switch 15, conductor 28, contact terminal 31, lever 41, conductors 53 and 51, contact terminal 49, lever 80, conductors 54 and 52, contact termina150, lever 81 and conductor 55 toconductor 2.4"- of circuit- This -will cause switch '15to operate and close a 'circuit, as follows fromone conductor of "circuit '10, conductor 18, the contact terminals andcontact bridging member offswitch 15*, conductor 14,-the primary winding 'of transformer 20 and conductor 11, to the other conductorof circuit "10.- A

still larger number of turns of the primary winding will be included in-the circuit, and, therefore, the secondary voltage willbe still smaller. Within normal limits, the secondary volta e should now;be low enou hto: permit'only such current to flow throng ;the resistor as will 'maintain the temperature at a constant value; If, however, the temperature risesstill further, resulting in a further decrease of resistanca and, hence, an increase of current, the derived current in' the heating coils, and especially in the heating coil 66", will become large enough to cause the strip (SG tobe-diSe'ngaged from the contact terminal and to engage contact terminal 67. This action will result in closing a new control circuit, as follows: from conductor 24 of circuits-24, through conductor 74, strip 66%, contact terminal.67, conductor 60,-coil 35,1conductors 59 and 28, contact terminal 31,lever 41, conductors 53 and 51, contact terminal 49, r lever 80, conductors 54 and'52,-contact terminal 50, lever 81 and conductor 55, to conductor 24 of circuit 24. This will cause relay 25 to operate, and lever 41' vwill engage contact terminal 43, interrupting the circuit through the coil of switch 15, causing it to operate and interrupt the circuit through the 'primary'wind ing of transformer 20. The transformer will, therefore be denergized and the furnace 21, as well .as theheating coil 66", will begin to cool, and, aftera predetermined lower temperature has been reached, the

strip 66 -will be disengaged from contact terminals 67 and engage contact terminal 65.

The control circuit through the coil of switch 15 will be reestablished, and this switch will operate to close the circuit through conductor 14, thereby reenergizing the transformer 20 and causing current to again flow throu h the resistor.

If it is desire to change the operating temperature of the furnace, the rheostat 'i 9 may be adjusted to accomplish this object. As the ad'usting handle of the rheostat is moved to ecrease the resistance included in the circuit in shunt with the heating coils, more current will be diverted from the heating coils, and, therefore, the current through theresistor of furnace 21 must be greater before the thermostats will operate in the ma'nnerdescribed above. Switch 19 may be used to connect the two halves of the secondary winding of transformer 20 in parallel or in series to enable extreme voltage requirements to be met, and is not a part of my invention.- a a It is clear that various modifications and changes may be made,without. departing from =thespirit and :scope of my invention, and I desireithat only such limitations shall be placed thereon as are imposed by the prior art or are specifically set forth in the appended claims.

claim as my invention:

. 1. In an electrical heating system, the combination with a variable source of energy and a heating element having a negative-temperature-resistance coeflicient the temperature of which is to be regulated, a supply-circuit, a transformer having a plurality-of spaced taps on the primary winding,-and a plurality of electromagnetically operated switches for controlling the connections of the supply circuit to the primary winding of the transformer, of a plurality of wplural-position switches for actuating said electromagnetically operated switches, and temperature-actuated means for controlling said plural-position switches.

2. In an electrical heating system, the combination with a main heating element having a negative-temperature-resistanee coefiicient the tem erature of which is to be regulated, a supp y circuit, a transformer having a plurality of spaced taps on the primary winding, and a plurality of electromagnetlcally operatedswitches for controlling the connection of the supply circuit to the primary winding of the transformer, of a plurality of plural-position switches for actuating said electricallyoperated switches, temperature-controlled means for controlling said plural-position switches, and an auxiliary heating element out of thermal communication with the main heating element for actuating said temperature-controlled means.

3. In an electrical heating system, the combination with a main heating element ha ng a n g t te emper u -re ta ce coeflic ent, a supply circuit, a' transformer having a plurality of spaced taps on the primary winding, and a plurality of electrically-operated switches for controlling the connection of the sup lyicircuit to the primary winding of said transformer, of plural-position switches ,for actuating said electrically-operated, switches, thermostats for controlling saidplural-position switches and an auxilla ry heati ielement ,out of thermal communlcation w th themain heat-- ing element for actuating said thermostats.

4. In an electrical heating system, the combination with a mainyheating element, having; a negative-temperature.-resistance coefiicient, a supply circuit, v a" transformer having a plurality of spaced taps on the primary winding, and a pluralityof electrically operated a switches for ;-controlling ,the connections of thesupplyi'cire'uit vto the primary winding of. the. itransiormen iofaplurality, of plural-.position switches or ,causing said electrically. :operated zswiteh to. operate yin redetermined order thermostats for actuatmg saidaplural-pos tion sw-it es .and an auxiliary; heating 1 element out 10f thermal communication with} ithe main heating element for ectuatingsaid thermostats. e

- 5. In an; electrical -heating,-circuit,= the combination with a main heating element havin a negative temperature :1- resistance. coefficient, a supply-circuit, a transformer having a plurality of-spaced taps on the primary winding,and a plurality ofelectrically. operated switches for controlling the cone nection of the supply circuit to the primary winding of said tra'nsformer 'of plura1- contact relays for eausingsai electrieally operated switches toqoperate Lon :predet'er mined order, a plurality'iof.double-contact; thermostats for actuating said; :relays, and! an auxiliary heating element out of thermal communicationwit the 'main heating element for actua said thermostats 6. In an electmcalheating' system, the combination .with a main -,-,heating elementhavi a negativestemperature-resistance coefliclent, a supply circuit,-a transformer having anadjustable primary winding, and, a plurality of electroma etlcally operated switches for-controlling t e-number of turns of the primary windin connected to said supply circuit, of plum -contaetrelays for operating said electromafnetically operated switches, a plurality of ouble-contact ther mostats for actuatmg said relays, and an auxiliary heating element having a pinrality of sectionswof diiferent-heating ca pacities out of thermal communication with the main-heating elementfor actuating said thermostats i i 7. In an electrical "heating system, the combination with amain'heating element having a negative-temperature-resistanee ooeiiicient,Ia supply circuit, a transformer ingaprima andia secondar winding, said a g P3 51) and a se ry d l} primar win ing being provi ed with spaced said primary winding. being provided wlt 'taps and said secondary windin being conspaced taps and said secondary winding. benected to the main heating cement, and

' ing connected to said main heating element, electroma notic' switches'govermng the conand electromagnetic switches governing the nection o the supply circuit to said taps, of connection of the supply circuit to said plural-contact, lural-coil relays for contaps, of plural-contact relays for causing ,trolling said eectromagnetic switches, a said electrical switches to vary the number plurality of double -contact thermostats of turns of the primary winding connected adapted to controlsaid relays, and an auxin sa d s pplyt a plurality of b1 i-liary, heating element out of thermal comcontact thermostats for actuating said remunication with 'the main heating element laiys, and an auxiliary heating l m nt out for actuating said thermostats in predeter- 0 thermal commumcati n w th th m in mined order, whereby a substantiallytconheating element and receiving current in 5mm fiul'lent is m in in i mam proportion to, and derived from, the curhefltin" l men rent in said main heating element for actu- B an l tri al heating system, the ating said th rmo t t combination with a main heating element 8. In an electrical heating system, th hav nga negativetemperature-resistance cocombination with a main heating element Ffllclentz a pp y i atfansfolmei having a negative-temperature-resistance g p y Wmdmg P' with Spaced coefiicient, a supply circuit, a transformer taps and a secondar winding connected to haying i a y i di d a Secondary said main heating e ement, and a plurality winding said primary wmdin being of electromagnetic switches for selectively videdwith spaced taps and said secondary connecting 531d taps to Bald PPP 011mm, winding being connected to said main heat of P r t c Plural-C011 ye for ing element, and'electromagnetic switches controllmg i f electromagnetic Swlbches, governing the connection of the supply cirm n p i ng a hermostat, for actuatcuit to sald taps, of plural-contact relays for g 0116 Of Sald ay 9 Cause Its 9 causin said electromagnetic switches to e g electromagnetic swltch to op an to as varyt e number of turns of the primary p f p the 9 mm through h prima y winding connected to said supply circuit a w g ransformer, and an auxplurality of double-contact thermostats or ihary heating co 1l for controlling said theractuating said relays, an auxiliary heating m0mt Sflld auxlllery e g c011 g element out of thermal communication with erglzfi y a current Y from, and R the main heating element and receiving cur- Portlonal the current 111 the [B91111 heatmg rent in proportion to, and derived from, the element d arranged to cause 531d thermo' current in said main heating element for e t0 operate Whenever the Current thIOII h actuating said thermostats, and means for Sald mam heatlllg element exceeds a P varying the proportion between the two curtermined Vallle- 106 rent values. 12. In an electrical. heating element, the

9. In an electrical heating system, the Combination t a main ng element combination with a main heating element havmg a g e: mp r i having a negative-temperature-resistance cofi' n a supply olrcult, a transformer efiicient,a supply circuit,atransformer havhaving a primary winding provided With 11;) mg a primary and a secondary winding, said spaced taps and a secondary winding conprimary winding being provided with nected to said main heating element, and a spaced t d th secondary i di plurality of electromagnetic switches for being connected to said main heating eleselectively connecting said taps to the supment, and electromagnetic switches governply circuit, of plural-contact, plural-coil re- 1'15 in th nn ti f th u l i it to ays for controlling said electromagnetic said ta s, of plural-contact, plural-coil reswitches, a double-contact thermostat for lays or causing said electromagnetic actuating one of said relays, an auxiliary switches to progressively increase the numheating element for controlling said ther ber of turns of the primary windin conmostat, said auxiliary heating element be nected to the supply circuit, a plurality of ing energized b a current erived from, double-contact thermostats adapted to conand proportions to, the current in the 'main trol said relays, and an auxiliary heating heating element, and arranged tocause said element out of thermal communication with thermostat to actuate one of said relays to the main heating element for actuating said cause its cooperating electromagnetic switch thermostats in predetermined order. to interru t the circuit through the pri 10. In an electrical heating system, the mary win ing of said transformer when; combination with a main heating element ever the current through said main heating having anegative-temperature-resistance coelement exceeds a predetermined amount efficient, a supply circuit, a transformer havandtojrestore the circuit through the primary winding after a predetermined lower temperature is reached.

13. In an electrical heating system, the combination with a main heating element having a negative tPll'lpflIatllle resistance coefficient, a supply circuit, a transformer havin a primary 'inding provided with spacer taps and a. secondar i winding connected to said main heating eleiniit, and a plurality of electromagneturswitches gov erning the connection of said supply circuit to said taps, of a plurality of double-contact, double-coil, relays for controlling the operation of said electromagnetic switches, said relays being electrically interlocked to preclude more than one electromagnetic switch being closed at one time, a plurality of double-contact thermostats adapted to actuate said relays, and an auxiliary heating coil out of thermal communication with the main heating coil for controlling said thermostats. Y

14. In an electrical heating system, the combination with a variable source of energy and a main heating element having a negatiye temperature resistance coefiicient adapted to be energized from said source of energy, of means for selectively varying the effect of said source of energy on said heating element, said means comprising a plurality of plural-position relays, thermostats for controlling said relays, and an {tutt iliary heating, element out of thermal communication with the main heating element a ud receiving current in proportion to, and derived from, the current in said main heating element for actuating said thermostats.

15. In an electrical heating system, the combination with a variable source of energy and a main heating element having a negative temperature-resistance coeflicient adapted to be energized from said source of energy, of means for selectively varying the effect of said source of energy on said heating element, said means comprising a plurality of plural-position relays thermostats for controlling said relays, an auxiliary heating element out of thermal communication with the main heating element and receiving current in proportion to, and derived from, the current in said main heating element for actuating said thermostats, and means for varying the proportion between the two current value In testimony whereof, I have hereunto siibscrilied my name this seventh day of March, 1919.

FRANK THORNTON J 2;. 

